Many broadband focusing x-ray optics take advantage of total reflection at glancing angles of incidence. Total reflection occurs when the angle of incidence at the entrance or opening of the x-ray optic is less than a critical angle that depends upon the properties of the reflecting material and the x-ray energy. This angle is referred to herein as the opening or acceptance angle. This category of x-ray optic is referred to herein as a grazing incidence multi-shell optic (GIMSO).
Many GIMSO designs have used metal, glass or plastic substrates with coatings of nickel, gold or iridium at glancing angles ranging from 10 to 150 arc minutes. Double-reflection geometries of the Wolter-I or Kirkpatrick-Baez types have been developed to focus a parallel beam of x-rays. The Wolter-I configuration typically consists of confocal paraboloid-hyperboloid shells and has been used most often for x-ray telescopes designed for high angular resolution. This optic is relatively axially compact, has a moderate field of view and, in some cases, a large number of surfaces can be nested to fill a substantial fraction of the available entrance aperture. An approximation to the Wolter-I design replaces the precisely figured optics with simple cones. Telescopes based upon this approximation have been developed for various astrophysical payloads. The Kirkpatrick-Baez geometry uses two parabolic surfaces for parallel-to-point focusing, and it has been adapted to point-to-point geometries for x-ray microscopes.
Another GIMSO design includes a surface shaped into a cylindrical spiral for single reflection, point-to-point focusing. The spiral surface may be a ribbon of smooth plastic coated with any one or combination of metals such as nickel, gold or iridium, or other suitable materials (e.g., high Z materials), and may be coated with multiple layers of such materials. Instead of a spiral, such a GIMSO may be formed from concentric cylinders of the same material. Other configurations of metal coated plastic may be used as well to guide, focus and/or concentrate x-rays.
Another category of optics for focusing x-rays are capillary optics typically formed from bundles of capillary tubes. In such capillary bundles, the x-rays undergo numerous reflections as they travel through the glass channels. The individual capillaries typically have lower efficiency than the GIMSO type optics discussed above and typically have significantly shorter focal lengths. However, the extremely large number of capillaries per solid angle of collection makes the ultimate throughput of the capillary system relatively high, and may have relatively large opening or acceptance angles as compared to GIMSO type optics. While to capillary optics are typically formed from glass tubes, capillary optics may be formed from any type of suitable material, and the term capillary optic refers herein to any optic formed from a collection of capillary tubes of any suitable material. Typically, capillary optics guide x-rays using multiple reflections (e.g., 5, 10 or even hundreds or more reflections).